Array Access and Update at Index in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to access the element at index 2 in the array.

DSA Java

int[] numbers = {10, 20, 30, 40, 50};
int value = numbers[[1]];
System.out.println(value);

Drag options to blanks, or click blank then click option'

A1

B3

C2

D4

Attempts:

3 left

💡 Hint

Common Mistakes

Using index 3 instead of 2, which accesses the fourth element.

Forgetting that array indexes start at 0.

The index 2 accesses the third element in the array, which is 30.

2fill in blank

medium

Complete the code to update the element at index 1 to 99.

DSA Java

int[] data = {5, 10, 15, 20};
data[[1]] = 99;
System.out.println(java.util.Arrays.toString(data));

Drag options to blanks, or click blank then click option'

A0

B3

C2

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Updating index 0 instead of 1.

Using an index out of array bounds.

Index 1 refers to the second element, which is updated to 99.

3fill in blank

hard

Fix the error in the code to correctly update the last element of the array.

DSA Java

int[] arr = {2, 4, 6, 8};
arr[[1]] = 10;
System.out.println(java.util.Arrays.toString(arr));

Drag options to blanks, or click blank then click option'

A1

B3

C2

D4

Attempts:

3 left

💡 Hint

Common Mistakes

Using index 4 which is out of bounds.

Confusing array length with last index.

The last element is at index 3 because array indexes start at 0.

4fill in blank

hard

Fill both blanks to access and update the element at index 0 to 100.

DSA Java

int[] values = {7, 14, 21};
values[[1]] = [2];
System.out.println(java.util.Arrays.toString(values));

Drag options to blanks, or click blank then click option'

A0

B1

C100

D50

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong index like 1 or 2.

Assigning wrong value like 50 instead of 100.

Index 0 accesses the first element, which is updated to 100.

5fill in blank

hard

Fill all three blanks to update the element at index 2 to 77 and print it.

DSA Java

int[] nums = {11, 22, 33, 44};
nums[[1]] = [2];
System.out.println(nums[[3]]);

Drag options to blanks, or click blank then click option'

A1

B2

C77

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Using different indexes for update and print.

Assigning wrong value instead of 77.

Index 2 is updated to 77, then printed from the same index.

Practice

1. What does the expression arr[2] do in a Java array named arr?

easy

A. Deletes the element at index 2

B. Adds a new element at index 2

C. Accesses the element at index 2 of the array

D. Returns the length of the array

Solution

1. Step 1: Understand array indexing

   In Java, arrays use zero-based indexing, so arr[2] accesses the third element.

2. Step 2: Identify the operation

   The expression reads the value stored at index 2 without modifying the array.

3. Final Answer:

   Accesses the element at index 2 of the array -> Option C

4. Quick Check:

   arr[2] means get element at index 2 [OK]

Hint: Remember arrays start at index 0, so arr[2] is third element [OK]

Common Mistakes:

• Thinking arr[2] adds or deletes elements
• Confusing index with array length
• Assuming indexing starts at 1

2. Which of the following is the correct syntax to update the element at index 1 of an integer array nums to 10?

easy

A. nums(1) = 10;

B. nums[1] = 10;

C. nums{1} = 10;

D. nums.1 = 10;

Solution

1. Step 1: Recall Java array update syntax

   In Java, array elements are accessed and updated using square brackets: array[index] = value;

2. Step 2: Match correct syntax

   Only nums[1] = 10; uses correct brackets and assignment.

3. Final Answer:

   nums[1] = 10; -> Option B

4. Quick Check:

   Use square brackets for array index update [OK]

Hint: Use square brackets [] to access or update array elements [OK]

Common Mistakes:

• Using parentheses () instead of brackets []
• Using curly braces {} which are invalid for arrays
• Trying to access with dot notation

3. What will be the output of the following Java code?

int[] arr = {5, 10, 15, 20};
arr[2] = 25;
System.out.println(arr[2]);

medium

A. 25

B. 15

C. 20

D. 10

Solution

1. Step 1: Understand initial array values

   The array arr starts as [5, 10, 15, 20].

2. Step 2: Update element at index 2

   We set arr[2] = 25; changing the third element from 15 to 25.

3. Step 3: Print updated element

   System.out.println(arr[2]); prints the updated value 25.

4. Final Answer:

   25 -> Option A

5. Quick Check:

   arr[2] updated to 25, so output is 25 [OK]

Hint: Remember updates change the element before printing [OK]

Common Mistakes:

• Printing old value before update
• Confusing index 2 with index 3
• Assuming array is unchanged

4. Identify the error in the following code snippet:

int[] numbers = {1, 2, 3};
numbers[3] = 4;

medium

A. ArrayIndexOutOfBoundsException at runtime

B. Syntax error due to missing semicolon

C. Compilation error: invalid array declaration

D. No error, code runs fine

Solution

1. Step 1: Check array size and valid indices

   The array numbers has length 3, valid indices are 0, 1, 2.

2. Step 2: Analyze index used for update

   Attempting numbers[3] = 4; accesses index 3, which is out of bounds.

3. Step 3: Identify error type

   This causes a runtime ArrayIndexOutOfBoundsException.

4. Final Answer:

   ArrayIndexOutOfBoundsException at runtime -> Option A

5. Quick Check:

   Index 3 invalid for length 3 array [OK]

Hint: Array indices go from 0 to length-1 only [OK]

Common Mistakes:

• Assuming index 3 is valid for length 3 array
• Confusing compile-time and runtime errors
• Ignoring zero-based indexing

5. Given the array int[] data = {2, 4, 6, 8, 10};, which code snippet correctly doubles the value at index 3 and then prints the entire array?

hard

A. data[4] = data[4] * 2;
for(int i = 0; i < data.length; i++) {
System.out.print(data[i] + " ");
}

B. data[3] *= 2;
for(int i = 1; i <= data.length; i++) {
System.out.print(data[i] + " ");
}

C. data[3] = data[3] + 2;
for(int i = 0; i < data.length; i++) {
System.out.print(data[i] + " ");
}

D. data[3] = data[3] * 2;
for(int i = 0; i < data.length; i++) {
System.out.print(data[i] + " ");
}

Solution

1. Step 1: Identify correct index and operation

   Index 3 corresponds to the fourth element (value 8). Doubling means multiplying by 2.

2. Step 2: Check loop correctness for printing

   The loop must run from 0 to data.length - 1 to print all elements without error.

3. Step 3: Verify each option

   data[3] = data[3] * 2; for(int i = 0; i < data.length; i++) { System.out.print(data[i] + " "); } correctly doubles data[3] and loops from 0 to length-1. data[3] *= 2; for(int i = 1; i <= data.length; i++) { System.out.print(data[i] + " "); } uses invalid loop bounds and will cause runtime error. data[3] = data[3] + 2; for(int i = 0; i < data.length; i++) { System.out.print(data[i] + " "); } adds 2 instead of doubling. data[4] = data[4] * 2; for(int i = 0; i < data.length; i++) { System.out.print(data[i] + " "); } doubles wrong index (4).

4. Final Answer:

   data[3] = data[3] * 2; for(int i = 0; i < data.length; i++) { System.out.print(data[i] + " "); } -> Option D

5. Quick Check:

   Correct index and loop bounds ensure proper update and print [OK]

Hint: Use index 3 for 4th element; loop from 0 to length-1 [OK]

Common Mistakes:

• Using wrong index for update
• Incorrect loop bounds causing errors
• Adding instead of multiplying